ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-11-9643-2011Biological residues define the ice nucleation properties of soil dustConenF.1MorrisC. E.2LeifeldJ.3YakutinM. V.4AlewellC.11Institute of Environmental Geosciences, University of Basel, 4056 Basel, Switzerland2INRA, Unité de Pathologie Végétale UR407, 84140 Montfavet, France3Air Pollution/Climate Group, Agroscope Reckenholz-Tänikon Research Station ART, 8046 Zürich, Switzerland4Institute of Soil Science and Agrochemistry, Siberian Branch of the Russian Academy of Sciences, 630099 Novosibirsk, Russia16092011111896439648This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/11/9643/2011/acp-11-9643-2011.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/9643/2011/acp-11-9643-2011.pdf

Soil dust is a major driver of ice nucleation in clouds leading to
precipitation. It consists largely of mineral particles with a small
fraction of organic matter constituted mainly of remains of micro-organisms
that participated in degrading plant debris before their own decay. Some
micro-organisms have been shown to be much better ice nuclei than the most
efficient soil mineral. Yet, current aerosol schemes in global climate
models do not consider a difference between soil dust and mineral dust in
terms of ice nucleation activity. Here, we show that particles from the clay
and silt size fraction of four different soils naturally associated with 0.7
to 11.8 % organic carbon (w/w) can have up to four orders of magnitude
more ice nucleation sites per unit mass active in the immersion freezing
mode at −12 °C than montmorillonite, the nucleation properties of which
are often used to represent those of mineral dusts in modelling studies.
Most of this activity was lost after heat treatment. Removal of biological
residues reduced ice nucleation activity to, or below that of
montmorillonite. Desert soils, inherently low in organic content, are a
large natural source of dust in the atmosphere. In contrast, agricultural
land use is concentrated on fertile soils with much larger organic matter
contents than found in deserts. It is currently estimated that the
contribution of agricultural soils to the global dust burden is less than
20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere
of a very different and much more potent kind than mineral dusts.